Define the limiting reagent.

• A. The reactant that is completely consumed first, limiting the amount of product formed.
• B. The reactant present in the smallest amount initially.
• C. The reactant with the highest molar mass.
• D. The reactant that is consumed last, determining the extent of the reaction.

Answer: (A)

Limiting reagent is the reactant that is consumed first, and it fixes the maximum amount of product that can form. To determine which one it is, compare how much of each reactant you have to how much is required by the balanced equation. For each reactant, divide the number of moles you have by its coefficient in the equation; the smallest value points to the limiting reagent because that reactant runs out first as the reaction proceeds. The smallest initial amount isn’t automatically the limiter, since the coefficients dictate how much of each reagent is needed relative to the others. Molar mass doesn’t affect limiting behavior, and the reagent that is consumed last is not the limiter—the others run out first, setting the limit on product formation.